Wherever there’s light and an object, there’s a shadow. By blocking sunlight, Earth casts a shadow, too.
But can you see Earth’s shadow? Astronomers told Live Science that yes, you can see it cast on several objects. In fact, under the right conditions, you may even spot Earth’s shadow every day.
Shadow on the moon
Because the sun is an extended source of light (meaning it isn’t a single point of light), Earth’s shadow has three parts: a dark central umbra, a lighter peripheral penumbra, and an antumbra that appears at faraway distances where overlapping penumbral regions replace the umbra.
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Earth’s shadow is the most prominent during total lunar eclipses. During a lunar eclipse, the full moon passes through Earth’s shadow. The moon enters Earth’s penumbra, and then its umbra, before passing through the penumbra again. Distinguishing Earth’s penumbra is difficult, since the moon dims only a bit. In contrast, the umbra, which can be up to 2.7 times the lunar diameter, is much easier to see.
However, when the moon is entirely ensconced in Earth’s umbra, it typically isn’t fully black. Most times, it’s a faint to deep red. That’s because our planet’s atmosphere scatters sunlight and then bends these reddened beams into the umbra.
“In a way we are seeing sunrises and sunsets on Earth reflected off the Moon!” Emily Rice, an astronomy professor at Macaulay Honors College of the City University of New York told Live Science in an email.
The shadow can also tell us about Earth’s atmosphere. “The cloudier and dustier the Earth’s atmosphere at the time, the more red the Moon will appear,” she noted.
In comparison, seeing a blackened moon is possible when the moon straddles Earth’s umbral-penumbral boundary. This stems from a contrast effect, since the portion of the moon in the penumbra is 500 times brighter than in the umbra, astrophysicist Fred Espenak wrote on EclipseWise.
Earth’s shadow is also visible in partial lunar eclipses, when only part of the moon passes through the umbra.
Daily delight
But you don’t have to wait for an eclipse to see Earth’s shadow. Immediately before sunrise and just after sunset, our planet’s silhouette — cast into space — becomes visible in the sky near the horizon in the direction opposite the sun.
“During the day, the clear sky is bright because direct sunlight illuminates the gas molecules and particles of our atmosphere, which can scatter some of this sunlight,” Raymond L. Lee, adjunct professor in the Mathematics and Science Division at the U.S. Naval Academy, told Live Science in an email. “As the sun sets or rises, the [roughly] spherical earth blocks the lowest solar rays from illuminating the atmosphere directly, thus casting a curved shadow on the corresponding lowest parts of the atmosphere that are opposite the sun.”
However, the exact features constituting Earth’s shadow are debated. A 2017 study in the journal Applied Optics says the shadow comprises an upper dark-blue band and a lower brown band. But Giovanni di Giovanni, an amateur astronomer who has a master’s degree in physics from the University of L’Aquila, in Italy, said neither is the real shadow. Instead, our planet’s shadow is “a thin, very dark band, but with little contrast compared to the other bands above it,” he told Live Science in an email.
In any case, the shadow is present when the sun is just above the horizon until it’s about 4 degrees below it. The planet’s silhouette persists for about 15 minutes, before it disappears with the rising sun or merges with the darkening night sky.
Seeing Earth’s shadow requires a clear, dust-free sky at high elevation. “Very few observers would notice it … only from the tops of high mountains, and perhaps with less difficulty from an airplane,” di Giovanni said.
Earth’s shadow on other objects
Spotting Earth’s shadow on other objects is possible but trickier. If the International Space Station passes overhead on a cloudless evening at sunset, a pair of stargazing binoculars can help you see it dim as it passes into Earth’s shadow.
Additionally, telescopes can show how geostationary satellites “vanish” in Earth’s shadow. Such satellites orbit Earth directly above the equator, at the same rate as the planet rotates, making them seem fixed in the sky. According to satellite operator Space Norway, geostationary satellites are mostly bathed in sunlight. But that changes for approximately 21 days around every equinox. According to the Australian Space Weather Forecasting Centre, during these “eclipse seasons,” the satellites slip into Earth’s shadow for several minutes each night.
Earth’s shadow even appears on passing asteroids. For instance, astronomer Gianluca Massi, founder of The Virtual Telescope Project, wrote in a blog post that one of their telescopes recorded the house-size asteroid 2016 VA dimming and disappearing as it passed through Earth’s shadow for 10 minutes, 50 seconds at a distance of 74,520 miles (120,000 kilometers).
But there’s a limit to how far Earth’s shadow extends. “The [Earth’s] umbra is a very long cone shape that extends about 1.4 million km (870,000 mi) away from Earth,” Rice said. So, Earth’s shadow won’t reach Mars, which, at its closest, is 34 million miles (55 million km) away. Still, with lots of objects that pass through Earth’s shadow, there are many ways to keep seeing — and marvelling — at our planet’s silhouette.
